Protein materials are generally present in the tissues of many biological species. Therefore, the development of medical devices that utilize protein materials, which mimic and/or are biocompatible with the host tissue, have been pursued as desirable devices due to their acceptance and incorporation into such tissue. For example the utilization of protein materials to prepare drug delivery devices, tissue grafts, wound healing and other types of medical devices have been perceived as being valuable products due to their biocompatibility potential.
The use of dried protein, gelatins and/or hydrogels have previously been used as components for the preparation of devices for drug delivery, wound healing, tissue repair, medical device coating and the like. However, many of these previously developed devices do not offer sufficient strength, stability and support when administered to tissue environments that contain high solvent content, such as the tissue environment of the human body. Furthermore, the features of such medical devices that additionally incorporated pharmacologically active agents often provided an ineffective and uncontrollable release of such agents, thereby not providing an optimal device for controlled drug delivery.
A concern and disadvantage of such devices is the rapid dissolving or degradation of the device upon entry into an aqueous or high solvent environment. For example, gelatins and compressed dry proteins tend to rapidly disintegrate and/or lose their form when placed in an aqueous environment. Therefore, many dried or gelatin type devices do not provide optimal drug delivery and/or structural and durability characteristics. Also, gelatins often contain large amounts of water or other liquid that makes the structure fragile, non-rigid and unstable. It is also noted that the proteins of gelatins usually denature during preparation caused by heating, the gelation process and/or crosslinking procedures, thereby reducing or eliminating the beneficial characteristics of the protein. Alternatively, dried protein devices are often very rigid, tend to be brittle and are extremely susceptible to disintegration upon contact with solvents. The deficiencies gelatins and dried matrices have with regards to rapid degradation and structural limitations make such devices less than optimal for the controlled release of pharmacologically active agents, or for operating as the structural scaffolding for devices such as vessels, stents or wound healing implants.
Hydrogel-forming polymeric materials, in particular, have been found to be useful in the formulation of medical devices, such as drug delivery devices. See, e.g., Lee, J. Controlled Release, 2, 277 (1985). Hydrogel-forming polymers are polymers that are capable of absorbing a substantial amount of water to form elastic or inelastic gels. Many non-toxic hydrogel-forming polymers are known and are easy to formulate. Furthermore, medical devices incorporating hydrogel-forming polymers offer the flexibility of being capable to be implantable in liquid or gelled form. Once implanted, the hydrogel forming polymer absorbs water and swells. The release of a pharmacologically active agent incorporated into the device takes place through this gelled matrix via a diffusion mechanism.
However, many hydrogels, although biocompatible, are not biodegradable or are not capable of being remodeled and incorporated into the host tissue. Furthermore, most medical devices comprising of hydrogels require the use of undesirable organic solvents for their manufacture. Residual amounts of such solvents could potentially remain in the medical device, where they could cause solvent-induced toxicity in surrounding tissues or cause structural or pharmacological degradation to the pharmacologically active agents incorporated within the medical device. Finally, implanted medical devices that incorporate pharmacologically active agents in general, and such implanted medical devices comprising hydrogel-forming polymers in particular, oftentimes provide suboptimal release characteristics of the drug(s) incorporated therein. That is, typically, the release of pharmacologically active agents from an implanted medical device that includes pharmacologically active agent(s) is irregular, e.g., there is an initial burst period when the drug is released primarily from the surface of the device, followed by a second period during which little or no drug is released, and a third period during which most of the remainder of the drug is released or alternatively, the drug is released in one large burst.
Also, particles made from decellularized tissue, such as human, bovine or porcine tissue, have also been utilized in various medical applications. These decellularized tissue particles have been utilized in various applications as subcutaneous tissue fill materials. Furthermore, these substances have been shown to have some biocompatible properties, but generally are difficult to work with due to the already established matrix present in such materials. Furthermore, such tissue related materials are not conducive to the homogenous distribution of pharmacologically active agents within their matrix structure.
Additionally, other polymeric materials, such as polyvinyl pyrrolidone, polyvinyl alcohols, polyurethanes, polytetrafluoroethylene (PTFE), polypolyvinyl ethers, polyvinylidene halides, polyacrylonitrile, polyvinyl ketones; polyvinyl aromatics, ethylene-methyl methacrylate copolymers, polyamides, polycarbonates, polyoxymethylenes, polyimides, polyethers and other polymeric materials have been utilized as coatings for medical devices, drug delivery devices, tissue fillers or grafts, sutures and for other medical applications. These materials possess some biocompatible attributes, but are limited by their capacity to be non-thrombogenic, to be non-inflammatory, to allow direct cell integration, to deliver therapeutic agents, to allow regeneration of host tissue into the graft and/or to allow other graft materials to adhere to their surface.